This invention relates to the field of spectrum analysis, and more particularly to the field of measurements of spectral features in a spectrum analyzer display using a cursor and a marker.
Spectrum analyzers are electronic instruments that convert time domain information at their input into frequency domain information at their output. Time domain information describes how the amplitude of an electrical signal varies with time, while frequency domain information describes how the power of the signal is distributed over frequency.
Prior art spectrum analyzers have used markers and/or cursors to assist in spectral measurements. Typically, one marker is placed on the peak of a signal and the frequency and amplitude of the signal at that point are displayed somewhere around the screen. Some spectrum analyzers provide two or more markers that can be used to measure differences between the amplitude and frequency at one point and the amplitude and frequency at another point.
The 49X series of Spectrum Analyzers from Tektronix, Inc. has markers that can be linked together to perform bandwidth measurements. In one mode of operation, these markers automatically go to the -6 dB points on either side of a peak, and a readout displays the frequency difference between the locations of the two markers. Markers can be moved individually or as a linked pair automatically by NEXT PEAK .fwdarw. and .rarw. NEXT PEAK controls. The instrument locates the center of the next peak in the indicated direction and moves the single marker to the top of it or the pair of markers to the -6 dB points on either side of it.
The R3261/3361 Spectrum Analyzers from Advantest also have two markers, which may be linked, and a horizontal cursor. A readout associated with the cursor indicates the amplitude level at the cursor's location and the difference in amplitude levels between the cursor's location and the location of the primary marker. The two markers are linked for bandwidth measurements by a "X dB DOWN" softkey. When the cursors are linked, they must be positioned on the peak of interest and a button pressed to activate a measurement. Each activation produces a single measurement. When one marker is used in conjunction with the cursor, the difference in amplitude levels between the location of the marker and the location of the cursor is displayed. This instrument also has automatic peak searching via .rarw. NEXT PEAK .fwdarw. controls, but there is no way to link the cursor to the markers or to make the linked markers move from peak to peak to perform a sequence of bandwidth measurements.
Also, in the Advantest R3261/3361 Spectrum Analyzers, if the frequency span or center frequency is changed, the markers do not track the moving peak. There are prior art spectrum analyzers from Tektronix, e.g. the 2710 or the 49X analyzers, that automatically track a peak when it or the frame of reference moves, and perform a new calculation as soon as the real or apparent motion of the peak stops for a second or two. However, in all of these spectrum analyzers with markers that can be linked to perform bandwidth measurements, fluctuations in the spectrum being measured can cause the locations of the markers to move up and down vertically by a considerable amount that depends on the steepness of the peak being measured and the resolution of the display.
Because prior art spectrum analyzers place markers based on analysis of amplitude axis information, they must have relatively stable conditions to locate a peak, find the X dB down points and put the markers on them, and then make the frequency axis measurement. It would be preferable to find an approach that would allow measurements to be made continuously, even under more dynamic conditions, such as when signals are fluctuating or instrument settings are being adjusted.
What is desired is an improved marker and cursor system for spectral waveform measurement that permits the operator to link or unlink a cursor and a marker, allowing them to be used separately or in coordination, and that provides a second marker so that additional measurements can be performed when the first marker and the cursor are being used linked together, and that performs bandwidth measurements in a way that keeps the X dB down points stable in the vertical axis despite fluctuations in the shape of the peak being measured, and that continues measuring even under dynamic conditions.